The voice quality of a conference call depends on the quality of the conference bridge as well as the quality of the access lines and terminal equipment used by the participants. Often, the quality of the access lines and/or terminal equipment is the limiting factor. It is common that a poor-quality access line or terminal equipment of a single participant ruins the quality of a large multi-party call. For example, a participant calling in from a cell phone with high background noise due to a poor radio signal will inject noise into the whole conference. In another example, a participant calling in by using a speaker phone with low echo return loss can generate excessive echo for the whole conference and make it difficult for everyone else to listen or speak.
To maintain the quality of a conference call, various mechanisms have been put in place at conference bridges. Conference bridges usually include echo cancellers to cancel the echo returning from each individual port. Conference bridges often implement a volume-threshold which a participant's line volume-level must exceed before their signal is mixed into the conference's output audio stream. This mechanism is used to prevent the addition to the conference output stream of a large number of sources of background noise that would raise the conference's background noise level. Most conference bridges also implement some automatic gain control (AGC) mechanism to equalize the differences in levels between different participants.
These methods are usually applied to all ports of a conference call by static provisioning. Different processing parameters may also be administered for different port types or port groups, but once they are assigned, the processing parameters and strategies do not change with the particular conditions of individual legs of the conference call. While the processing algorithms are adaptive in nature (e.g., an echo canceller may adjust the cancellation-function's coefficient or an AGC may change the gain as the call progress), they do not exploit the full range of options of different strategies, such as, for example, choosing between linear echo-cancellation and half-duplex echo-suppression.